1. Field of the Invention
The present invention relates to a capacitive accelerometer, and more particularly, to an ultra-small capacitive accelerometer capable of detecting variation in capacitance between a reference electrode and a detection electrode by varying an area overlapping therebetween through a circuit to measure acceleration.
2. Discussion of Related Art
Ultra-small accelerometers are widely used in various fields such as automobile industries, defense industries, robot systems, safety diagnosis, and so on, due to their compact structure, low price, high performance, etc. Among these accelerometers, a capacitive accelerometer is a sensor for detecting variation in capacitance between a reference electrode and a detection electrode by acceleration generated by a force applied from the exterior to determine acceleration of a moving object.
The capacitance is determined by the following Formula 1. As a distance d between the electrodes becomes smaller and an overlapping area increases, the capacitance also increases.
                    C        =                  ɛ          ⁢                      A            d                                              [                  Formula          ⁢                                          ⁢          1                ]            
In addition, since variation in capacitance increases as variation in overlapping area increases, it is possible to manufacture a sensitive accelerometer. Therefore, when an ultra-small capacitive accelerometer is manufactured, it is important to increase variation in overlapping area.
FIG. 1A shows a conventional z-axis capacitive accelerometer having a cantilever beam connected to a support via a torsion part.
The accelerometer includes a substrate 100, detection masses 130 disposed at predetermined intervals and fixed to the substrate 100 by a support 110, and a torsion part 120 connecting the detection masses 130 to the support 110 and twisted by z-axial acceleration. Comb-shaped detection electrodes 140 are formed at ends of the masses, and reference electrodes 150 (150) overlapping the electrodes 140 are fixed to the substrate 100.
When an acceleration movement is performed by an external force in a direction perpendicular to the substrate 100, the detection mass 130 moves in a direction opposite to the acceleration direction as a support spring 120 is twisted, and thus, an area in which the detection electrodes 140 overlap the reference electrodes 150 varies. Therefore, capacitance between the electrodes varies so that the magnitude of acceleration can be detected therethrough. However, since the accelerometer moves only the detection mass with respect to the acceleration, variation in detection area is relatively small, and the accelerometer may be readily affected by the acceleration of the other axis.
FIG. 1B illustrates an example of a z-axis capacitive accelerometer of a seesaw structure having a detection mass disposed in a symmetrical manner with respect to a bending beam.
In FIG. 1B, a detection mass 160 is spaced apart from the substrate 100 by a bending beam 180 connected to a support 170 fixed to the substrate 100, and the mass is disposed in an asymmetrical manner with respect to the bending beam 180. A detection electrode is formed at both ends of the detection mass 160, and a reference electrode overlapping the detection electrode is fixed to the substrate 100.
In the accelerometer, when vertical acceleration is applied to the substrate, a heavy part 170 of the mass is raised or lowered due to the asymmetrical mass, and a light part 180 is moved in an opposite direction, representing a seesaw behavior. Here, the area in which the detection electrode overlaps the reference electrode varies so that variation in capacitance can be detected therefrom to detect acceleration. However, in this structure, similarly, the area varies by a displacement in which the asymmetric mass moves vertically. In addition, when a large mass is provided to increase sensitivity, affection of the acceleration of the other axis may be increased.